A grain-oriented electrical steel plate having an easy magnetization axis in a rolling direction of a steel plate has been used for an iron core of a power converter such as a transformer. A low core loss property has been required strongly for a material of the iron core in order to reduce loss to be caused at the time of energy conversion.
The core loss of an electrical steel plate is classified into a hysteresis loss and an eddy current loss roughly. The hysteresis loss is affected by a crystal orientation, a defect, a grain boundary, and so on. The eddy current loss is affected by a thickness, an electrical resistance value, a 180-degree magnetic domain width, and so on.
Then, in manufacturing the electrical steel plate, arts in which crystal grains are aligned highly in the orientation of (110)[001] and crystal defects are reduced have been employed in order to reduce the hysteresis loss. Further, in order to reduce the eddy current loss, arts in which a thickness of the electrical steel plate is thinned, an electrical resistance value is increased, and a 180-degree magnetic domain is subdivided have been employed. An increase in Si content or the like has been performed for the increase in the electrical resistance value, and coating of a tension film on a surface of the electrical steel plate or the like has been performed for the subdivision of the 180-degree magnetic domain.
In recent years, in order to reduce the core loss drastically, there also has been proposed an art in which in addition to the application of tension to the surface of the electrical steel plate in order to drastically reduce the eddy current loss, which occupies most of the core loss, a groove and/or a strain is/are artificially introduced into the surface of the electrical steel plate and further the 180-degree magnetic domain is subdivided.
For example, in Patent Document 1 and the like, there is described an art in which a laser beam is emitted in a direction perpendicular to a rolling direction of a surface of a grain-oriented electrical steel plate with a predetermined beam width and energy density, and at predetermined emitting intervals, thereby introducing a local strain into the surface.
In Patent Document 2, there is disclosed an art in which a groove is formed in a predetermined direction of a surface of a grain-oriented electrical steel plate with a predetermined load, and then fine crystal grains are generated in a strain introduction section by strain relief annealing.
In Patent Document 3, there is disclosed an art in which a groove having a predetermined depth is mechanically formed with a roller with a groove or the like in a predetermined direction of a grain-oriented electrical steel plate in which annealing has been performed, and thereafter by etching, fine grains caused by mechanical strain are removed to deepen the groove.
In Patent Document 4, there is disclosed an art in which grooves are periodically formed in a surface of a grain-oriented electrical steel plate in which a finish annealing film has been removed, and thereafter a tension film is applied thereto.
In Patent Document 5, there is disclosed an art in which an interval and an angle of a groove to be formed in a surface of a directional electrical steel plate are limited within a predetermined range.
These arts described in Patent Documents 1 to 5 presuppose that a film is formed on a surface of an electrical steel plate. That is, the formation of a film is indispensable.
However, there is sometime a case that a magnitude of tension of the film cannot be obtained sufficiently due to variation in manufacturing processes. Then, in the above case, a favorable core loss property cannot be obtained. As measures against this case, coating the film thickly is also performed, but thickening the film leads to an increase in a nonmagnetic layer inevitably, resulting that a magnetic flux density is lowered. Consequently, at the time of manufacturing a transformer, a necessity of using the electrical steel plate more is created, resulting that weight is increased and cost is increased.
Patent Document 1: Japanese Patent Application Laid-open No. Sho 55-18566
Patent Document 2: Japanese Patent Application Laid-open No. Sho 61-117218
Patent Document 3: Japanese Patent Application Laid-open No. 2000-169946
Patent Document 4: Japanese Patent Application Laid-open No. 2003-301272
Patent Document 5: Japanese Patent Application Laid-open No. Hei 7-320921